The cell biology and biochemistry of Leishmania is investigated as a model of both intra- and extra-cellular parasitism. As interactions between host and parasite occur at the level of the parasite surface membrane (SM), emphasis is placed on: 1) characterizing its biochemical and physiological functions; and 2) defining its roles in parasite survival and development. The SM proline-transport protein of L. donovani promastigotes was identified using both directed affinity matrix-binding and anti- idiotype antibody (anti-ID)-based methods. The anti-ID was generated against an affinity purified anti-polyproline IgG. The anti-ID immunoprecipitated the Leishmania SM protein and its also inhibited proline transport into intact promastigotes by greater than 50 percent. The SM acid phosphatase was partially purified and characterized. The biosynthetic and secretory mechanisms of the soluble acid phosphatase (SAcP) were defined and localized. Antigenic cross-reactivity was demonstrated amongst the SAcP's produced by promastigotes of various leishmanial species. Oligonucleotide probes were used to identify and clone the gene(s) for SAcP. The SM 3'-nucleotidase was purified to homogeneity and its amino terminus sequences. The SM 5'-nucleotidase was also isolated, purified and characterized. The current results provide further functional characterization of parasite SM constituents and these might serve as targets for the design of new chemotherapeutic agents and/or immunodiagnostic/ prophylactic reagents.